1. Field of the Invention
The present invention relates to a method for punching a hole in a sheet metal by press working and a press die.
2. Description of the Related Art
Conventionally, in a case where a hole is punched in a sheet metal by press working, it is the general practice to place a workpiece on a piercing die having a punching hole and subsequently to lower an upper die having a piercing punch so as to effect shearing work with respect to the workpiece by the punch and the die.
In the case where work is effected using such a conventional technique, it is normally necessary to control the clearance between the piercing punch and the piercing die to within plus 0.01 millimeter and minus 0.01 millimeter. If this clearance cannot be maintained, burrs can occur in the workpiece, or defects of punching edges of the piercing punch and the piercing die or the breakage of the piercing punch can occur owing to the interference between the piercing punch and the piercing die.
To prevent this problem from occurring, large cost of die fabrication and long periods of die design and fabrication are conventionally required, involving such as the planning of a die structure which makes it possible to ensure the axial accuracy of the piercing punch and the piercing die with high accuracy as well as manual operations for enhancing the die fabrication accuracy.
In addition, in a case where a hole is punched in a plate surface which is inclined with respect to the press working direction, since the piercing punch tends to escape in the direction of inclination to make it impossible to maintain the clearance, and since the breakage of the punch occurs, a slide cam mechanism is conventionally provided, and the punch is held by this slide cam mechanism to force out the punch in a direction different from the pressing direction.
Conventionally, as the aforementioned slide cam mechanism, if broadly classified, two structures have been devised.
One is a structure to hold a cam slider by a pad incorporated in an upper die holder, and this structure is effective to simplify the die structure. However, since this pad is held on the workpiece placed on the lower die, variation can occur in the position where the pad is held with respect to the lower die because of the variation of configurations of workpieces.
For this reason, this structure is not used since it is difficult to maintain the clearance between the piercing punch and the die necessary for the conventional punching method.
Accordingly, another slide cam mechanism is conventionally used in which a cam slider is mounted on the upper die holder or the lower die.
However, with this other slide cam mechanism, since not only is this cam mechanism itself expensive, but a multiplicity of dies are required for manufacturing a single pressed part, since it is impossible to effect the simultaneous cutting work of outer peripheral edges of a pressed part and the simultaneous piercing work of a plurality of different surfaces.
In addition, in the process of manufacturing pressed parts, there is a problem in that large repairing cost is involved for maintaining the axial accuracy of the piercing punch and the piercing die.
JP-A-11-129041 discloses a press die in which, instead of the piercing punch, a pressure medium typified by urethane rubber is pressed to punch a hole in a workpiece, to solve the above problems.
In the press die disclosed in that document, tension is applied to the workpiece at the hole portion of the piercing die buried in the lower die by means of the pressure applied to the pressure medium, and the workpiece is thereby fractured by an outer peripheral edge portion of the hole portion of the die so as to obtain a hole.
However, in the case where pressing is effected by the press die disclosed in that document, after the workpiece has been fractured by a portion of the outer periphery of the hole portion of the piercing die, the pressure applied to the pressure medium escapes to the fractured space, and the pressure medium is released. Therefore, even if further pressurization is effected, it is impossible to transmit tension for punching a hole in the workpiece, with the result that it is impossible to fracture over the entire region of the outer periphery of the hole portion of the piercing die, making it impossible to obtain a satisfactory hole configuration. In addition, there is another problem in that the pressure medium typified by urethane rubber is shaved off by the sharp edge portion of the piercing die hole, so that the pressure medium lacks durability.